The present invention relates to an engine brake control in an automatic transmission.
Automatic transmissions effect a shift between gears by a change-over in the power delivery path after selective actuation of one or more friction elements which may be of various kinds. It is the commonly employed practice to provide a one-way clutch which serves as a reaction member against a rotary member to establish a power delivery path.
With this one-way clutch, if the rotary member is subject to reverse torque, the one-way clutch is released to interrupt the transmission of this reverse torque. However, since the one-way clutch interrupts the transmission of reverse torque, no effective engine braking results.
Thus, a known automatic transmission is provided with a friction element which is arranged in parallel to a one-way clutch such that the friction element is activated to effect engine braking when a driver places a manual selector valve at a predetermined range position.
The engine brake control of this kind is employed for example in a H 700-R4 type transmission manufactured by General Motors Corporation. In this known transmission, as shown in diagram in FIG. 4, when a driver manipulates a manual lever 10 to place a spool of a manual selector valve 12 at an automatic shift drive range (D range) position as illustrated by the right half thereof as viewed in FIG. 4, line pressure P.sub.L from a circuit 13 is supplied to a circuit 14 leading to a shift valve 15. If, under this condition, shift pressure P.sub.S is available, a spool of the shift valve 15 is urged upwards to assume an upshift position as illustrated by the left half thereof where the hydraulic pressure from the circuit 14 is supplied to a circuit 16 leading to a shifting friction element 17 to activate the same to establish the nth speed (4th speed). If the shift pressure P.sub.S is not available, the spool of the shift valve 15 assumes a downshift position as illustrated by the right half thereof as viewed in FIG. 4 where the circuit 16 is connected to a drain port to discharge hydraulic fluid from the shifting friction element 17 to establish the n-1th speed (3rd speed) in cooperation with the action of a one-way clutch.
If, under the condition where the D range is selected, the driver wishes engine brake operation, he or she manipulates the manual lever 10 to place the spool of the manual selector valve 12 to a third speed engine brake range (III range) as illustrated by the left half thereof. In this position, line pressure P.sub.L from the circuit 13 is now supplied to a circuit 18 leading to an engine brake friction element 19 (an overrun clutch) to activate the same, causing engine braking during running with the n-1th speed (3rd speed). This friction element 19 is arranged in parallel to the above mentioned one-way clutch.
In order to prevent interlock which would be caused by engagement of friction element 19 while the friction element 17 remains engaged, the manual selector valve 12 is designed to drain fluid pressure from one of the circuits 14 and 18 before fluid pressure is supplied to the other.
Since in order to obtain engine braking the spool of the manual selector valve 12 has to move to the position where line pressure P.sub.L is supplied to the circuit 18 from the circuit 13, the number of ports of the manual selector valve 12 is increased and the stroke of the spool is also increased. This results in complicated manufacturing processes of the manual selector valve, and requires a complicated actuator system for operating the spool of the manual selector valve. Besides, in this known transmission, the manual selector valve has to be operated to effect engine braking. Thus, it is impossible to control engine braking in this transmission electronically.
An object of the present invention is to provide engine braking without relying on manipulation of a manual selector valve but still maintaining the function of preventing the above mentioned interlock from taking place.